Polarization-maintaining optical fibers have been conventionally employed as optical fibers for coupling optical components which exhibit polarization dependence.
In addition, such fibers are assembled into optical transmission components, e.g., fiber gratings and optical fiber couplers, for use in various optical transmission or measurement devices such as optical fiber gyroscopes.
For such polarization-maintaining optical fibers, polarization-maintaining optical fibers which are formed by providing stress-applying parts within a cladding at opposing sides with respect to a core to impart birefringence to the core are well known, as disclosed in U.S. Pat. No. 4,478,489 and Japanese Patent No. 2,750,345, for example. A polarization-maintaining optical fiber having such a structure are called a PANDA polarization-maintaining optical fiber (Polarization-maintaining AND Absorption-reducing optical fiber, hereinafter referred to as a “PANDA fiber”), and one of the advantageous characteristics of PANDA fibers are that PANDA fibers having an excellent polarization-maintaining performance can be manufactured with high precision.
Polarization-maintaining optical fibers having a cladding 125 μm in diameter are commonly used since such fibers are easy to manufacture, can be manufactured with a uniform quality, and easily couple to conventional optical fibers used for transmission. During a typical manufacturing process of polarization-maintaining optical fibers, a proof test has been conventionally conducted where predetermined tension is applied to fibers to be tested and broken ones are removed.
A trend toward high-capacity and high-density optical communications and the advent of high-precision optical measuring instruments have created a strong demand for reducing the size of optical transmission components and measurement apparatuses. Furthermore, an enhancement of cooling efficiency of optical transmission components and measurement apparatuses by reduction in size is also required because such devices generally include one or more heat-generating units.
However, when conventional polarization-maintaining optical fibers having a cladding 125 μm in diameter are bent with a small bending radius below 20-30 mm, problems of deteriorated performance, e.g., increased microbend loss, and compromised reliability, e.g., increased tendency to breakage, will arise. A larger bending radius is thus required when optical component are coupled using this type of optical fiber, which poses a challenge inhibiting downsizing of optical components.
As an example of polarization-maintaining optical fiber which can be bent with a bending radius smaller than 20 mm, a polarization-maintaining optical fiber having a small-diameter cladding of 80 μm used for gyroscope is known, as reported in “Polarization-maintaining Optical Fiber”, Fujikura Technical Review, No. 85, October 1993.
For reducing microbend loss, this polarization-maintaining optical fiber for gyroscope is designed so that the fiber has a larger relative refractive index difference Δ between the core and the cladding (e.g., 0.8-1.2%) (hereinafter just referred to as “relative refractive index difference”), and has a smaller mode field diameter (MFD) (e.g., between 3 μm and 5 μm at a wavelength of 0.85 μm, and between 5.5 μm and 7.5 μm at 1.55 μm) than conventional polarization-maintaining optical fibers.
However, a smaller mode field diameter is disadvantageous since splice loss due to an axial offset incurred during fusion splicing tends to increase. In addition, when a fiber having a smaller mode field diameter is spliced with a telecommunication optical fiber having a cladding 125 μm in diameter, e.g., an optical fiber for transmission, splice loss due to a difference in mode field diameters becomes significant.
Although polarization-maintaining optical fiber for gyroscopes can be manufactured based on the aforementioned structural parameters because such fibers are not to be coupled with another optical fiber, splice loss is a critical issue in a polarization-maintaining optical fiber used for coupling or other applications. Therefore, structural parameters of polarization-maintaining optical fibers for gyroscope cannot be used for polarization-maintaining optical fibers used for coupling or other applications.